Waveguides can be used for electromagnetic power transmission. In particular, the geometry and physical-electromagnetic properties of a waveguide may determine modes of electromagnetic propagation corresponding to persistent or semi-persistent solutions to Maxwell's Equations, subject to boundary conditions of the waveguide. Such mathematical solutions are referred to formally as “eigenmodes,” and represent resonant modes of the waveguide that can give rise to standing waves within the waveguide, a volume customarily referred to as the waveguide “cavity.” Introducing or injecting electromagnetic energy into a waveguide cavity in such a way that “excites” or “couples to” an eigenmode causes power to be transmitted efficiently within the waveguide, without a physical transmission medium, such as wires.